Papers by Author: Nak Sam Choi

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Authors: Sung Choong Woo, Dae Joon Kim, Nak Sam Choi
Abstract: Acoustic emission (AE) characteristics have been studied for single-edge-notched monolithic thin aluminum (Al) plates and glass fiber/Al hybrid laminates. Traveling microscope was used for observing the plastic deformation and damage zone around the initial notch tip. Frequency characteristics of AE signals processed by fast Fourier transform (FFT) from monolithic Al could be classified into two different types. Type I signal had a relatively low frequency band of 96~260kHz, while Type II signal had broad band frequencies of 192~408kHz. In case of glass fiber/Al hybrid laminates, AE signals with high amplitude (>80dB) and long duration (>2msec) were additionally confirmed on FFT frequency analysis, which corresponded to macro-crack propagation and/or delamination between aluminum layer and glass fiber layer. Also, distributions of the first and the second peaks in frequency spectrum were related with local fracture behaviors of the hybrid laminates. AE source location determined by signal arrival time showed the extent of fracture zones. On the basis of the above AE analysis, characteristic features of fracture processes of single-edge-notched glass fiber/aluminum laminates were elucidated according to different fiber orientations.
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Authors: Sung Hyuk Lee, Nak Sam Choi
Abstract: To analyze the bending collapse behavior of an aluminum square tube under the bending moment load, a finite element simulation for the four-point bending test has been performed. Using an aluminum tube beam specimen partly inserted with two steel bars, local buckling deformation near the center of the tube beam was induced. Simulated moment-rotation angle curve obtained during the post-collapse period of the aluminum tube with steel bars were in good agreement with experimental result, which was comparable to the result obtained from Kecman's theory. Using a combination of the four-point bending test and its finite-element simulation, analysis of the local buckling and the bending collapse behavior of an aluminum tube beam could be quantitatively accomplished.
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Authors: Sung Choong Woo, Nak Sam Choi
Abstract: Dominant frequency characteristics of acoustic emission (AE) from single-edge-notched (SEN) glass fiber aluminum laminates (GFAL) under tensile loads were analyzed in relation to fracture mechanisms. The first and second peak frequencies expressed the characteristic changes of fracture processes in SEN-GFAL specimens such as macro-crack propagation and/or delamination between aluminum and fiber layers. On the basis of the above frequency analysis and of the fracture observation with ultrasonic through-scan and various microscopes, characteristic models for crack propagation of SEN-GFAL was proposed according to various orientations of fiber layer.
901
Authors: Dal Woo Jung, Il Bum Kwon, Nak Sam Choi
Abstract: A temperature-compensating fiber Bragg grating(FBG) sensor having two different FBGs in one fiber line was applied to the real time measurement of mechanical normal strain in structures. Measurement of mechanical strains of the aluminum beam surface by the double FBG sensor was performed under various thermal conditions, which was compared with results of electrical resistance strain gage. The FBG sensor fabricated in this study could detect accurately values of mechanical strains without containing any thermal strain component.
1089
Authors: Sung Hyuk Lee, Cheol Woong Kim, Nak Sam Choi
Abstract: Bending collapse behaviors and energy absorption characteristics of aluminum-GFRP hybrid tube beams were evaluated by using experimental tests combined with theoretical analysis. Hybrid tube beams composed of glass fiber-epoxy layer wrapped around on aluminum tube were made in autoclave with the recommend curing cycle. The hybrid tube beams showed a considerable improvement in their bending performance. The maximum bending moment and specific energy absorption of the hybrid tubes were higher than those of the aluminum tubes. They were also evaluated as a function of ply orientation and thickness of GFRP layer. A modified theoretical model was developed to predict the resistance to the collapse of hybrid tube beams subjected to a bending load. Theoretical ultimate bending moments and moment-rotation angle curves of hybrid tube beams were in good agreement with experimental ones. Hybrid tube beams strengthened by GFPR layer with 90°/0° and 45°/-45° ply orientation showed an excellent bending strength and energy absorption capability, respectively. Therefore, on the basis of above results, it was concluded that aluminum-GFRP hybrid tube beams might be employed as reinforcing and/or energy absorbable light weight space frame.
1825
Authors: Sung Hyuk Lee, Hyung Jin Kim, Nak Sam Choi
Abstract: Bending deformation and energy absorption characteristics of aluminum-composite hybrid tube beams have been analyzed for improvement in the bending performance of aluminum space frame by using experimental tests combined with theoretical and finite element analyses. Hybrid tube beams composed of glass fabric/epoxy layer wrapped around on aluminum tube were made in autoclave with the recommended curing cycle. Basic properties of aluminum material used for initial input data of the finite element simulation and theoretical analysis were obtained from the true stress-true strain curve of specimen which had bean extracted from the Al tube beam. A modified theoretical model was developed to predict the resistance to the collapse of hybrid tube beams subjected to a bending load. Theoretical moment-rotation angle curves of hybrid tube beams were in good agreement with experimental ones, which was comparable to the results obtained from finite element simulation. Hybrid tube beams strengthened by composite layer on the whole web and flange showed an excellent bending strength and energy absorption capability.
769
Authors: Sung Hyuk Lee, Nak Sam Choi
Abstract: Bending performances of aluminum square tube beams reinforced by aluminum plates under three point bending loads have been evaluated using experimental tests combined with theoretical and finite element analyses. Basic properties of aluminum materials used for initial input data of the finite element simulation were obtained from the true stress-true strain curves of specimens which had bean extracted from the Al tube beams. True stresses were determined from applied loads and cross-sectional area records of a tensile specimen with a rectangular cross-section by real-time photographing. True strains were obtained from in-situ local elongation measurements of the specimen gage portion by the multi-point scanning laser extensometer. Four kinds of aluminum tube beam specimens adhered by aluminum plates were employed. The bending deformation behaviors up to the maximum load described by the numerical simulation were in good agreement with experimental ones. An aluminum tube beam strengthened by aluminum plate on the upper web showed an excellent bending capability.
2290
Authors: Dal Woo Jung, Nak Sam Choi
Abstract: Fatigue fracture behavior of a hybrid composite joint with riveting was evaluated in comparison to the case of static fracture. Hybrid composite joint specimens for shear test were made with layers of carbon fiber/epoxy composite and stainless steel. Characteristic fracture behaviors of those specimens were obviously different under static and cyclic loads. Static shear loading showed the fracture of a pure shear mode, whereas cyclic fatigue-shear loading caused the local stress concentration of a tensile mode and thus brought about the tensile fracture at that site. Experimental results obtained by static and fatigue tests were considered in modifications of design parameters of the hybrid joint.
1757
Authors: Seung Bum Kwak, Nak Sam Choi
Abstract: Coolant rubber hoses for automobile radiators under thermal and mechanical loadings can be degraded and thus failed due to the influences of contacting stresses of air, coolant liquid and to the locally formed electricity. In this study, degradation behavior of the radiator hose made of EPDM rubber was evaluated. The thermo-oxidative aging test showed that the surface hardness IRHD of the rubber increased together with a reduction of failure strain. By the electro-chemical test it was shown that the penetration of coolant liquid into the skin of the rubber hose arose inducing an increase in weight of specimens as well as a decrease in failure strain and IRHD hardness. The penetration of coolant liquid altered considerably the micro-structure and the micro-hardness distribution along the depth in the rubber hose. On the basis of the above results failure mechanisms of degraded EPDM rubbers were suggested according to the kinds of contacting stresses.
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